Multi-stage dual wall conduit hydroforming

Abstract: A method and apparatus capable of forming a dual tube conduit having a predetermined outer tube dimension, a predetermined inner tube dimension, and a predetermined gap between the tubes. A dual tube workpiece (W) is initially bent in one or more zones to a desired nonlinear configuration, the inner tube (50) and outer tube (52) are hydroform expanded simultaneously to obtain the selected inner tube dimension, and the outer tube is then hydroform expanded to the desired outer tube dimension and to the desired gap therebetween, while the inner tube is held constant.

Description: This invention relates to hydroforming of dual wall tubular products such as engine exhaust conduits, and particularly to hydroforming such products to obtain a uniform controlled spacing or gap between the inner and outer walls. In US-A-5 170 557 a hydroforming process is described for forming a dual wall conduit, with a minimum air gap between the inner and outer walls being assured. Often it is desirable to have a predetermined uniform air gap, not just a minimum gap, between the walls. Specifically, for example, an auto manufacturer may want a dual wall engine exhaust conduit with a 63.5 mm (two and one-half inch) outer wall diameter and a 57.15 mm (two and one-quarter inch) inner wall diameter, and a uniform spacing or gap between them. The prior known technology does not enable that to be assured. This is particularly so when the dual wall conduit is bent into various nonlinear configurations, as is usually done. Such bending tends to cause reduction in the conduit diameter, i.e., necking down of the walls, at the bend zones, and formation of wrinkles or bulges adjacent the bend zones. When subsequently hydroform expanded in the conventional way in which only the outer wall is expanded outwardly to the surface of the hydroforming die cavity, the result is an air gap of differing amounts and configurations along the conduit length. An object of this invention is to provide a method and apparatus capable of forming a dual tube conduit, even one with a nonlinear configuration, having a predetermined outer tube dimension, a predetermined inner tube dimension, and a predetermined uniform desired spacing or gap between the tubes. The invention is set out in the independent claims. The dual tube workpiece is initially bent in one or more zones to the desired nonlinear configuration, the inner and outer tubes are hydroform expanded simultaneously to obtain the selected inner tube dimension, and the outer tube is then hydroform expanded to the desired outer tube dimension and to the desired gap or spacing therebetween, while the inner tube is held constant. The inner tube has openings along its length, specifically adjacent to and spaced somewhat from the ends thereof. During the time that both the inner and outer tubes are expanded simultaneously, these openings are sealed with an end plug seal so that hydroforming fluid only enters the inner tube to expand both tubes to a first selected dimension. This not only gives a controlled expansion but also reverses the necking down characteristic and the wrinkles caused by the prior bending step, and gives the inner tube the dimension required. Moreover, if there is a flaw in the inner tube, e.g., in the longitudinal seam weld of the tube, it will be detected at this stage because hydroforming fluid will escape between the tubes through the flaw and then squirt between the tubes at the ends of the workpiece, causing the pressure to noticeably drop or cease to build. Leakage of the tube will thus be apparent. Once this first expansion step is complete, the openings in the inner tube are purposely uncovered, i.e., unsealed, and hydroforming fluid is then reinjected under pressure into the inner tube, thus flowing through the openings into the outer tube, to expand the outer tube to the selected larger size, while the inner tube remains fixed with pressure being equal on both sides thereof. Hence, both tubes will then be at the selected dimensions and the spacing or gap between them will be the desired amount, which can be uniform over the length and configuration of the conduit. The fluid is then extracted from the conduit by drainage or by forcing it out under pressure. To accomplish this process, preferably a pair of forming mold cavities are used, the first cavity having a size larger than the initial workpiece and of a size to govern the inner tube final dimension, and the second cavity of a size larger than the first cavity and selected to establish the outer tube final dimension. These first and second cavities are each formed by having part of the cavity in one platen and the other part in a cooperative mold platen. When the platens are brought together, this completes and closes the cavities. The first mold cavity may also function as a die, i.e., when the platens are brought together to close the cavity, the cavity walls may work the metal of the workpiece to reshape it somewhat. Although one pair of end plugs can be used for both cavities, it is normally more convenient to have two pairs of end plugs. The first pair requires a resilient, radially expandable annular seal to seal off the openings in the inner tube of the workpiece. The first pair also preferably has tapered ends to flare the workpiece ends and thereby establish locating centres, i.e., the centreline of the workpiece. The second pair of end plugs has tapered ends of a nature to not only centre the workpiece in the second die cavity, but also to press and seal the flared ends of the inner and outer tubes together sufficiently during the second hydroforming stage to prevent leakage therebetween. The first pair of end plugs has a dual actuator arrangement to operate the tapered end for flaring of the workpiece, and to subsequently activate the annular seal. The invention may be carried into practice in various ways but one form of hydroforming apparatus and the method by which the apparatus is used, both in accordance with the invention, will now be described by way of example with reference to the accompanying drawings, in which: Fig. 1 is a plan view of the hydroforming apparatus, showing first and second die cavities and first and second pairs of end plug subassemblies; Fig. 2 is an enlarged, elevational view of one of the first pair of end plug subassemblies; Fig. 3 is a fragmentary sectional view of an end portion of the workpiece after the ends are flared; and Fig. 4 is a diagrammatic elevational view of the hydroforming apparatus. The complete assembly 10 includes a pair of cooperative platens 12 and 12', the lower one 12 being optionally a mirror image of the upper one 12'. These define a first mold cavity 14 and a second mold cavity 16. The diametral and circumferential dimensions of the first cavity are smaller than those of the second cavity, and are sized to provide a desired final dimension for the inner tube of the workpiece. The diametral and circumferential dimensions of the second cavity are sized to the desired final dimension of the outer tubular member of a pair of tubular members forming the workpiece, to be described in more detail hereinafter. Cavity 14 has a configuration from end to end matching that of the desired final conduit, especially a vehicle engine exhaust conduit, configured to match the requirements of a particular vehicle and shown, for example, to have a pair of bend zones between the opposite ends thereof. The bend zones in these two forming cavities 14 and 16 correlate with each other positionally. Using the present technology, a previously bent exhaust pipe conduit workpiece W is first placed in cavity 14, operated upon, and then placed in cavity 16 and operated upon further. At the opposite ends of the first cavity 14 is a first pair of special end plug subassemblies 20. Each of these is shown in more detail in enlarged fashion in Fig. 2. Each includes a frustoconical, tapered nose 22 oriented toward the cavity, and having a diameter which varies from the smallest diameter outer end portion, smaller in diameter than the diameter of cavity 14 and the inside diameter of the inner tube, to the largest diameter portion which is larger than the diameter of cavity 14. Each tapered nose is shiftable axially on the central axis of subassembly 20 for extension and retraction, by a first power actuator 24, preferably a fluid cylinder, with nose 22 being attached to the piston rod of the cylinder. Tapered nose 22 on the two end plugs is for the purpose of flaring the ends of the conduit workpiece W inserted in cavity 14, and holding the workpiece on centre in the cavity. End plug subassembly 20 also includes a radially expandable annular, deformable, resilient seal 28 mounted around a central rod 30 which has an enlarged flange-type collar 32 on its outer end and against the axial outer end of seal 28. The other axial inner end of seal 28 abuts against collar 34 adjacent the outer end of tapered nose 22. This entire assembly can be axially advanced by a fluid cylinder 25 into the cavity and workpiece, or retracted therefrom. The other fluid cylinder or actuator 24 has a short stroke to shift collar 34 axially outwardly to compress and axially squeeze resilient seal member 28, causing its outer diameter and inner diameter to radially expand, and thereby seal the ends of the workpiece. The at-rest smaller diameter of seal 28 is purposely made smaller than the interior diameter of workpiece W, while the expanded diameter is equal to, or even slightly greater when unrestrained, than the inner diameter of the workpiece, to form a fluid tight seal therein and against rod 30 for purposes to be explained hereinafter. These annular seals extend sufficiently into the workpiece to seal off openings 54 from the inner ends of the end plugs. Extending through end plug subassemblies 20 to communicate with a workpiece in cavity 14 is a liquid conducting passage 26 for entry and exit of hydroforming fluid such as water, as explained more fully hereinafter. The second pair of end plug subassemblies 40 for second cavity 16 is also characterized by having a tapered, frustoconical nose 42, the smaller end diameter of which is oriented toward cavity 16, and is smaller in diameter than this second cavity 16, while the larger diameter portion is larger in diameter than the diameter of cavity 16. A fluid cylinder power actuator 44 axially shifts the end plug with its tapered nose toward and away from cavity 16. In the second pair of end plugs 40, at least one has a liquid conducting passage 46 therethrough into the modified workpiece W' in cavity 16 for filling and pressurizing hydroforming liquid, normally water, in this workpiece, in a manner to be described more fully hereinafter. The initial workpiece to be hydroform-expanded comprises an inner, metal, preferably steel, and most preferably stainless steel, tube or tubular element 50, and an outer tubular element 52, also of metal, and preferably steel, most preferably stainless steel. The inner diameter of outer tube element 52 basically coincides with the outer diameter of inner tube element 50 such that normally the initial workpiece has 360° contact between the two elements along the length thereof. The inner element has at least one opening 54 extending through its wall thickness from the inner cavity 56 defined by the inner element to the inner wall of the outer element. The one or more openings along the length of the inner element are located only adjacent one end or both ends, preferably both ends, of the inner element, spaced from the open ends of the element an amount to be inward of the tapered noses 22 when in the first cavity, and inwardly of tapered noses 42 when in the second cavity. The tube elements of the initial workpiece are typically cylindrical in configuration, not yet having the flared end portions depicted in the drawings. Conceivably, however, the ends could be previously flared prior to placement in the first hydroforming cavity, e.g., when the tubes are pulled or rammed together or when the double tube is bent to effect any desired nonlinear configuration or angles therein. Furthermore, some double wall conduits or conduit portions need not have any bend zones, such that the cavities would have straight centrelines. If the ends are previously flared, it is still desirable to have tapered noses on the end plug for the first cavity, to hold the tubes on centre in the cavity. The opposite ends 16' of cavity 16 are outwardly tapered to match the configuration and angle of the tapered noses 42. Optionally, the opposite ends of cavity 14 may also have outwardly flared portions matching those of the tapered noses 22. However, it is not as necessary to have these tapered ends on cavity 14 as on cavity 16 since the interaction of the tapered noses 42 and the ends 16' of cavity 16 must function to seal between the two tube elements 50 and 52 of the workpiece at the flared ends, as described hereinafter, during the second hydroforming stage of the process. The purpose of the two-stage hydroforming operation is to first expand or enlarge both the inner and outer tube elements simultaneously by hydroforming in first cavity 14, and thereby obtain a predetermined final inner tube dimension, and then subsequently to expand or enlarge by hydroforming only the outer element further, while not changing the size of the inner element, using the second cavity 16. The workpiece is typically bent by conventional techniques to the overall desired configuration, e.g., like that shown with two angles as in Fig. 1. This workpiece is at least mostly of smaller outside diameter than the diameter of cavity 14 and is laid in the lower part of the cavity 14, and the top platen 12' is brought down to interfit with lower platen 12. During this closing, portions of the workpiece can be partially formed by the walls of cavity 14 acting as a die. High pressure is used to hold the platen totally closed and fluid actuators 25 are shifted axially to extend the first end plug subassemblies 20 into the workpiece W and the cavity 14. Specifically, the tapered nose elements 42 are forced toward cavity 14, thereby engaging the cylindrical ends of workpiece W and flaring them outwardly as the tapered noses extend to their final position partially within cavity 14. This flaring enables the workpiece to be held on centre in this cavity and also in the subsequent cavity 16. When actuator 25 inserts nose 22, it also inserts seal 28 into the cavity 14 and the workpiece a predetermined distance, past the openings 54 of inner tube 50. The other power actuators 24 are then actuated to axially extend collar 34 a small amount, thereby axially compressing the resilient annular seals 28. This causes them to radially expand into tight engagement with the ends of the inner peripheral wall of inner tube element 50, as well as with the rod 30, to tightly seal the ends of the inner workpiece cavity 56 axially inwardly of openings 54. Hydroforming liquid is then injected through liquid conduit 26 in at least one of the end plug subassemblies to fill space 56, while extracting the air as through a second passage 26' in the opposite end plug subassembly. The hydroforming process may be performed in a bath of liquid, e.g., water, so as to be submerged. In such a situation, filling of the workpiece will occur with submersion of the workpiece so that only a small amount of added liquid under pressure through passage 26 will be necessary for hydroforming. Sufficient pressure is then applied to the liquid to simultaneously expand both the inner and outer tubular elements 50 and 52 until the outer element outer surface takes the configuration and size of cavity 14, to give the inner element its desired final dimension. At this first forming stage, any flaws, e.g., in the weld of the longitudinal seam of inner element 50, can be detected since the pressurized liquid inside cavity 56 will tend to flow through any flaw in inner element 50 to be between tube elements 50 and 52 and thus squirt out of the ends of the workpiece between the elements, causing the hydroforming liquid pressure to noticeably drop or cease to build. This first step thus acts as an excellent quality check on the inner element. The pressure is then released, seals 28 are allowed to radially retract by retracting collar 34 axially, and the end plugs with tapered noses 22 and seals are retracted from the modified workpiece W' and cavity 14. If the operation is performed under liquid, i.e., in a bath of the hydroforming liquid, there is no need to drain the workpiece when it is transferred over to second cavity 16. If the operation is not performed in a bath, then the liquid is preferably drained from the workpiece prior to transfer of the workpiece over to the second cavity. This can be done by applying air pressure, or by gravity. Inasmuch as the size, i.e., diameter, of the second cavity is greater than that of the first cavity, there will be some slack between the outer wall of the transferred, expanded workpiece W' and the peripheral wall of the second cavity. The end plug subassemblies 40, when axially extended, cause the second pair of tapered noses 42 to engage the flared end portions of the workpiece to thereby centre it in cavity 16. Prior to hydroforming pressure being applied in the second operation stage, upper platen 12' is tightly closed with lower platen 12. When the operation is to be performed in the second cavity, another workpiece W can also be placed in the first cavity and operated upon in the manner just described. At the same time, the tapered noses 42 of the second pair of end plug subassemblies 40 are inserted into cavity 16 and the workpiece W' with sufficient force to press the flared ends of inner and outer elements 50 and 52 tightly together to create a seal between them. This is to prevent hydroforming liquid from escaping between the two tube elements during the second hydroforming operation. In this stage, openings 54 are now exposed to the entire inner cavity 56 of the workpiece. Therefore, when hydroforming liquid is injected to fill space 56 and then a significant forming pressure is applied in the workpiece, the liquid will flow through openings 54 such that the pressure on both the inner wall and the outer wall of inner element 50 is equal, but there is a significant outward pressure and force on the inside wall of outer element 52; causing it to expand to the selected dimensions of cavity 16, giving the outer element its desired dimension. After this is performed, the pressure is released and the forming liquid is drained out of the workpiece, or forced out under pressure, to empty the workpiece of liquid. Optionally, the offal at the ends of the workpiece, i.e., the flared end portions, can then be severed to leave the finished conduit product. It will be appreciated that various minor changes may be made to the process and apparatus described, for example to accommodate a particular type of material, configuration or product use. One such variation would be to not flare the ends of the workpiece as preferred but to otherwise form the seal at both ends. Another variation would be to use only one cavity but with removable inserts or shiftable walls, to render the cavity smaller for the first hydroforming stage and larger for the second hydroforming stage. Another variation would be to have the first and second cavities in separate platens.

Claims: A method of forming a configurated dual tube air gap metal conduit with a spacing between the inner and outer tubes comprising: providing a dual tube metal workpiece (W) having an inner tube (50) and an outer tube (52) in engagement with each other; placing the dual tube workpiece in a mold cavity (14); filling the inner tube with fluid and increasing the pressure of the fluid in the inner tube sufficiently to cause the inner and outer tubes to both expand a controlled amount in the mold cavity to a desired size of the inner tube; placing the workpiece in a larger mold cavity (16); pressurizing fluid in the inner tube, and between the inner and outer tubes, to equalize pressure on the inside and the outside of the inner tube, and expand only the outer tube to a final desired outside shape and size corresponding to the larger mold cavity, and causing a predetermined spacing of said outer tube from said inner tube, to thereby form a dual wall, air gap metal conduit. The method according to claim 1 which includes, prior to the second pressurizing step, outwardly pressing the ends of said inner and outer tubes together to seal the ends together to inhibit escape of pressurized fluid. A method according to claim 1 or claim 2 which includes the step of flaring the ends of the workpiece, preferably by forcing frustoconical noses (22) into the ends of the workpiece, and preferably prior to the first expansion step and preferably prior to the sealing of the open ends of the workpiece. A method according to claim 3 in which the noses (22) carry annular seals (28) and said sealing is achieved by radially expanding said annular seals. A method according to claim 4 in which said annular seals are resiliently deformable, and are axially compressed to cause the radial expansion. A method according to any of claims 3 to 5 which includes the step of cutting off said flared ends as offal. A method according to any of claims 1 to 6 which includes bending the workpiece (W) to a predetermined configuration prior to placing the workpiece in the first mold cavity (14). A method according to any of claims 1 to 7 in which the inner tube (50) has openings (54) therethrough placing said workpiece in the first cavity (14), closing the open ends of the tubes and also sealing the openings (54) off from said central space, and which includes, after placing the workpiece (W) in the second cavity, reclosing the ends of the tubes, but not sealing said openings (54). A method according to claim 1 which comprises providing the workpiece (W) of metal having a pair of ends, an inner peripheral tube (50) and an outer peripheral tube (52) in engagement with each other, and openings (54) through said inner tube adjacent to and spaced from said ends; providing a first pair of tapered end plugs (20) having an annular seal (28) on each said end plug, and providing a second pair of tapered end plugs (40); providing the mold cavity (14) larger than said workpiece, and providing the larger mold cavity (16) larger than said mold cavity (14) and dimensioned to match the final outer size of the conduit to be formed; placing said workpiece into said mold cavity (14); inserting said first pair of tapered end plugs into said workpiece ends to flare said pair of ends, and inserting said annular seals into said workpiece ends to a depth sufficient to cover said openings (54) and sealing said openings; filling said workpiece with fluid and increasing the pressure of said fluid sufficient to cause said inner and outer tubes to simultaneously expand a controlled amount to the size of said mold cavity (14); withdrawing said first pair of plugs from said workpiece; placing said workpiece in said larger mold cavity (16); inserting said second pair of tapered end plugs into said ends of said workpiece while leaving said openings uncovered, and forcing said inner and outer tubes together sufficiently at said workpiece ends, to inhibit fluid leakage from between said tubes; filling said workpiece with fluid; pressurizing said fluid to cause flow through said openings and between said inner and outer tubes, to expand only said outer tube to the size of said larger mold cavity (16) and a predetermined spacing from said inner tube, but not expanding said inner tube, and thereby form the dual wall metal conduit; and releasing the fluid pressure. A method according to claim 9 in which, during said step of inserting said second pair of tapered end plugs, said flared ends of said inner and outer walls are pressed together to seal said ends together. A method according to claim 9 or claim 10 including the subsequent step of cutting off said flared ends as offal. Hydroforming apparatus for forming a dual tube metal conduit of an inner tube (50) and an outer tube (52) having controlled spacing between the tubes, from an initial dual tube workpiece (W) having a pair of ends, engaging inner and outer walls, and openings (54) in said inner tube at the ends thereof, comprising: a forming cavity (16), said forming cavity having dimensions desired for the outer wall in the final conduit; a pair of tapered end plugs (40) at said forming cavity (16); and a power actuator for inserting said pair of end plugs into said workpiece ends without sealing said openings (54) in said workpiece, to cause pressurized fluid to flow through the openings and between the inner and outer tubes to expand only said outer tube to the size of said forming cavity (16), characterised by a further forming cavity (14) having dimensions smaller than those of said forming cavity (16), and a further pair of tapered end plugs (20) at said further forming cavity, having radially expandable seals (28) for insertion into the ends of a dual wall conduit workpiece, and having a hydroforming fluid inlet (26) and outlet (26') through said end plugs; a further power actuator for inserting said further pair of end plugs within the dual wall workpiece an amount sufficient to flare said ends of said workpiece and to cause said seals to cover said openings in said inner tube of said workpiece; and a fluid injector and pressurizer for injecting fluid through at least one of said end plugs, and pressurizing the fluid to simultaneously expand both said inner and outer tubes of said workpiece to the size of said further forming cavity. Hydroforming apparatus for forming a dual tube metal conduit having spaced inner and outer tubes (50 and 52) and controlled spacing between said tubes, from an initial dual tube workpiece (W) having engaging inner and outer tubes and openings in said inner tube at the ends thereof, comprising: a forming cavity (16) having dimensions desired for the exterior of the final conduit; sealing end plugs (20) oriented for insertion into the ends of a dual tube conduit workpiece, and having a hydroforming fluid inlet (26) through one said end plug, characterised by a further forming cavity (14), said further forming cavity having diametrical dimensions smaller than the forming cavity, said end plugs having a radially expandable annular seal (28), and one said end plug having an outlet (26') therethrough; power actuator mechanisms shiftable for inserting said end plugs into the dual tube workpiece an amount sufficient to cause said annular seal to close off the openings therein, and for activating a seal in the inner tube; a fluid injector and pressurizer for injecting fluid through said fluid inlet, and pressurizing the fluid to expand both said inner and outer tubes of said workpiece in said further forming cavity (14); said power actuator mechanism being shiftable to uncover said openings and allow pressurized fluid to flow through the openings and between the inner and outer tubes of the workpiece to expand only the outer tube in said forming cavity (16). Hydroforming apparatus according to claim 12 or claim 13 in which the or each power actuator mechanism comprises a pair of power actuators, one arranged to insert said end plugs, and the other arranged to expand said radially expandable axial seals. A hydroforming apparatus according to any of claims 12 to 14 in which said end plugs include flaring elements (22) oriented to flare and seal the ends of the conduit prior to flow of pressurized fluid between the inner and outer tubes.

Inventors: DEHLINGER JAMES R, RIGSBY DONALD R, WELLS GARY L, DEHLINGER, JAMES R., RIGSBY, DONALD R., WELLS, GARY L.
Applicants: BENTELER IND INC, BENTELER INDUSTRIES, INC.
Application year: 1994
Main classification: B21D26
IPC codes: B21D53, B21C37, B21D26, F01N13